The invention is based on a hydraulic anti-lock multicircuit brake system as defined hereinafter.
An anti-lock brake system of this kind is already known (German Offenlegungsschrift 37 42 173, FIG. 1), in which a relatively expensive brake pressure reducer, with which the brake pressure in the rear axle circuit can be reduced compared with the brake pressure in the front axle circuit beyond a pre-determined pressure level, is disposed in the brake circuit of the rear axle. Extending parallel to the brake pressure reducer is a bypass line having a multiposition valve for opening and closing that line. The electromagnetically switchable multiposition valve opens the bypass line whenever and as long as the brake pedal of the brake system is actuated, so that the brake pressure reducer cannot become operative. Brake pressure fed in by the master brake cylinder is therefore operative to its full magnitude in both the brake circuit in the front axle and the brake circuit of the rear axle. If in braking, the wheels of the rear axle threaten to lock, then the anti-lock control system of the brake system regulates the brake pressure at the wheel brakes of these rear wheels downward, so that there is no further tendency to locking and the stability of the vehicle is maintained. In a malfunction, such as a failure of the anti-lock control system, contrarily, the multiposition valve remains in the currentless state and blocks the bypass line. The communication from the master brake cylinder to the wheel brakes of the rear wheels is now via the brake pressure reducer. Not only is this known brake system expensive in design, but it also has less operational reliability, because the multiposition valve in the bypass line is under severe thermal strain if braking persists for relatively long periods.
A brake system is also known having an electronically controlled brake booster (German Offenlegungsschrift 34 40 541), which has an electromagnetically actuable multiposition valve in the brake line leading to the wheel brakes of the rear wheels; in the currentless state, this multiposition valve blocks the brake line. To control the distribution of braking force to the front axle and rear axle, the multiposition valve is switched over to the open position for certain periods of time as a function of the wheel rotation behavior, which is detected with the aid of sensors. Connected parallel to the multiposition valve is a valve assembly by way of which, if the multiposition valve or the energy supply fails or if the electronics malfunction, and so forth, brake pressure reaches the rear wheel brakes, this pressure being adapted to the minimum brake pressure as a function of the particular deceleration, given an ideal brake force distribution. For the purpose of brake force distribution, the embodiment of this brake distribution is also very complicated.